Where are you located? Humidity in the air will cause rusting, not sure that it's all that much more humid in your parking spot than the surrounding area. Plastic on top of the gravel will just slow evaporation, not stop it.
Much of the steel used to form an RV body isn't coated, it can cause more severe damaging rusting than untreated steel that rusts a bit, then the rust forms a barrier to future rust. You could try treating and undercoating everything under your RV, but I think it's a losing battle.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI